2 Answers
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I think you're trying to count how many slices of water there are in a river. I doubt you can come up with a good answer to the numerical portion of your question, but good luck trying.

As for any language being "officially considered a lingua franca," I would point out that even the original meaning of lingua franca had no official status. It was not a rule, regulation, or official designation; it was an observation that trade and diplomacy had developed a language (based mostly on Italian) that enabled effective communication among various peoples (whose native languages differed) in the eastern Mediterranean during Renaissance times.

I am aware of only one example of any language EVER being something like an "official" lingua franca, and it is this: By international agreement, ALL air traffic control communications throughout the entire world are conducted in English.

And here's an interesting related anecdote: In India, there are over 600 known languages, but almost every person there speaks English. As an Indian friend of mine said to me once, "If we didn't speak English, none of us would ever be able to talk to anyone else." It's not official, of course, but it's essential.

It depends what you mean by "officially". Most governments prefer to give their national languages priority, but they still usually teach English in their schools. And multi-national companies use English internally.

If yes, is there a way to find out what percentage of non-English populations that have English as the trade language, know English as a second language?

English is used as a trade language everywhere. (There are other trade languages, but they are only used within a region. When communicating with people from outside the region, English is the norm.) So we just need to calculate the percentage of the world's population who are non-native English speakers.

The world population is about 7 billion, and estimates of the number of non-native speakers range from &half; billion to 1 billion. You can do the math yourself.